1. Field of the Invention
The present invention relates to an image processing apparatus, image processing method, and a computer-readable storage medium for computer program for converting a color image into a monochrome image.
2. Description of the Related Art
Color documents are often printed in monochrome. Such instances, more often than not, involve a case where an available printer is a monochrome printer or a case where a monochrome printing mode is selected in a color printer to reduce a printing cost. In the monochrome printing, although colors of the document are not reproduced, a grey scale image having shades of gray according to the original colors can be printed. This makes it possible to distinguish, in a certain degree, differences in the original colors from the finished print.
On one hand, color document is printed in monochrome in this way. On the other hand, there are demands for colorization of the monochrome document. Among the purposes of colorization are improving the appearance of the document and improving legibility of the information. The colorization enhances legibility of the document information since a color image includes not only the information of lightness but also hue and saturation. For example, in a case where a document available at hand is a monochrome document converted from original colors, the monochrome document may have portions whose identification from one another is made difficult by the monochromatic conversion because two distinct original colors have been converted into densities (lightness) extremely close to each other. In such a case, colorization makes it possible to colorize again, using colors but may be different from the original, the portions used to be separated by different colors. As a result, such portions can be easily identified from one another.
There are following related arts that are known to propose colorization methods. JP-H10-324025A proposes a method for allocating predetermined colors to individual pixels of a monochrome image according to the density of the pixel. JP-2005-051510A proposes a method for colorizing each region of graphic or text so that the individual neighboring regions have different colors from each other. Also, JP-2006-350407A proposes a method for allocating colors by determining a photographic subject in a black and white picture. Further, application software that automatically performs colorization by referring to a color image having a pattern resembling a monochrome image to be colorized is in a practical use (Automatic Colorization Software “HighColor”, URL: http://www.recursion.jp/mitou15).